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TODAY'S CREATURE FEATURE!

10/24/06

Anemone Hermit Crabs

 

Anemone hermit crabs come in all shapes and sizes

Anemone hermit crabs

Subphylum: Crustacea

Class: Malacostraca

Order: Decapoda

Infraorder: Anomura

Family: Diogenidae and Paguridae

One of the most common animals we have been catching in our deep set baited epifauna traps are anemone hermit crabs (Dardanus brachyops, Dardanus pedunculatus and other species of the genus Dardanus with possibly two new species that we cannot identify. Hermit crabs are crustaceans, like lobsters, and have hard external skeletons and five pairs of legs. They are a very successful animal group with between 1500 and 2000 species around the world. Forty eight species are found in Hawaii, according to a recent report published by the American Fisheries Society, and they occur from the shoreline to depths of 1000 ft or more. Our traps are certainly proving the depth range to be true since we are catching them at 750 feet.  Many hermit crabs are nocturnal.  They are scavengers and omnivores, and they were likely attracted to our trap because of the dead mackeral the traps were baited with.

 

In this image you can see what the soft tail of the hermit crab looks like outside the shell. Photo: Joel Martin

In the Order Decapoda (shrimp, crabs, lobsters) the shrimp are the oldest form, and have been around for roughly 300 million years.  Lobsters are more recent and have the same body architecture as a shrimp, but are broader and more thickly shelled, and Hermit crabs (roughly 90 -100 million years old) have the same body form as a shrimp but have a soft asymmetrical abdomen (like the shrimp’s tail) rotated 90 degrees on its side. Hermit crabs use the shells of marine snails to protect these soft parts. The abdomens are adapted to curl into the spiral hole of a snail shell and grip the inside with the last pair of legs that are specifically designed for this purpose.  Once inside they are very difficult to remove, and can pull their entire body into the opening of the shell to protect themselves from danger or predators.  Many species also have a specially adapted oversize front claw, on the opposite side of the body from the curved tail, that can be used to close off the shell opening when the hermit retreats inside. Hermit crabs only leave their shell when they are changing to another, usually when they have outgrown their existing shell or have found another more suitable. 

 

An anemone with orange tentacles attached to a hermit’s shell.

Anemone hermit crabs differ from other hermits in that they take defense one step further by having a commensal (both organisms benefit) relationship with sea anemones attached to the outside of their shell.  Scientists know that anemone crabs will transfer anemones from one shell to another when switching shells, but they do not know how the anemones get on a newly inhabited snail shell. They do know that anemones are not present while the snail occupies the shell, so it has to have something to do with the crab.  One known species of anemone crab will actually eat the snail to get the shell, the rest will chose an empty shell, with several strongly attracted to the smell of a dying snail, like reading the obituary pages to find an apartment in Manhattan, or Honolulu for that matter.  There is even a case of an anemone/crab relationship where there is no shell involved, but the anemone secretes a shell like material that forms around the crab.  

Certain species of hermits are associated with certain species of anemones, and the most predominant anemone hermit we are finding, the jeweled anemone crab, Dardanus gemmatus, is typically associated with two species of anemones – Calliactis polypus and Anthothoe sp.  We may also have found a new species of anemone that was attached to the shell of an undescribed hermit crab. When threatened, the anemones are able to eject bright pink stinging threads (acontia) from their bases, and they hover above the crab like a defensive curtain.  The tentacles of the anemone sting as well, offering additional defense. The anemone is able to protect itself by withdrawing their tentacles, and closing up into a smooth surface.  Once the perceived danger has past they open up again and their tentacles wave in the currents.  The crab definitely gets added protection from this arrangement by associating with the stinging anemone, and the anemone may benefit by collecting scraps of food from the crab’s messy eating.   The anemones may also benefit from the mobility provided by the crab in the search for places where food is plentiful, and possible defense from predators on anemones. 

Anemone crabs are actually host to more than just anemones, and are a roving community of organisms.  The jeweled anemone crab has a flatworm (Stylochoplana inquilinia) and an amphipod (Elasmopus calliactis) associated with it, and on this trip a possible new species of barnacle was found on an anemone crab shell.

 

The specially adapted right claw of a box crab(left).

Photo: Joel Martin

Anemone crabs are eaten by several fish that predate upon crustaceans, such as puffer and triggerfish, as well as other animals that eat crustaceans.  Hermits may also be predated upon by a particular species of crab, the box crabs, family Callipidae, that have a special adaptation on one of their claws that allows them to chip open the shell of marine snails in the same fashion as a can opener. If a hermit has the bad fortune of being in one of these shells they may be eaten by the attacking box crab.  

References:

Hoover, John, “Hawaii’s Sea Creatures, A Guide to Hawaii’s Marine Invertebrates”

Personal conversations with Joel Martin, Gustav Paulay, Sea McKeon, and Cory Pittman

 

Anatomy of an Anemone Hermit Crab

Anemone hermit crab, Dardanus brachyops, with sea anemone, Caliactis armatus, caught in one of the deep set traps.  This particular animal also had a barnacle on its shell that the taxonomists could not identify, and it may be a new species.

Anemones: Anemone hermits often have more than one anemone on their shell, as well as more than one species of anemone that they are associated with.  This specimen took the prize for the most anemones.

Stinging Threads: The pink threads (acontia) in the image are composed of stinging cells, and the anemone ejects them for defense when it is threatened or stressed. These threads extend far beyond the anemone and may provide longer distance defense than the stinging cells in the tentacles themselves.

Legs for Grabbing Shell:  The hermit’s last pair of legs are adapted for grabbing holding onto the inside of the shell. Photo: Joel Martin

Eyes and Hairs: Hermit crabs have eyes that are on stalks so they can see beyond the edge of their shell.  The legs and body are also covered with short hairs that are chemosensory (smell) or tactile (they can feel, or sense movement in the water).

Other Associated Species: In addition to anemones, other species such as worms and amphipods are associated with anemone hermit crabs.  One crab we found had a barnacle, worm casings and anemones attached to its shell. This presents a particularly interesting evolutionary question of how these associations developed over time, and what the advantages are.

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Oversize Front Claw: Most hermit crabs have uneven front legs adapted as strong pincers.  Usually one claw is larger than the other and doubles as a strong claw for feeding and defense, and also to close the opening of the shell when the hermit retreats inside it.

 

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See New Species and New Records from French Frigate Shoals!

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*All images and information from French Frigate Shoals are provided courtesy of the Northwestern Hawaiian Islands Marine National Monument, Hawaiian Islands National Wildlife Refuge, the Northwestern Hawaiian Islands State Marine Refuge, and NOAA's Pacific Islands Fisheries Science Center in accordance with permit numbers NWHIMNM-2006-015 and NWHIMNM-2006-01.